a. Field of the Invention
This invention relates to a method for utilizing regrind polyethylene terephthalate (PET) for the production of disposable plastic cups, lids, clam-shell type trays, and containers. More particularly, the present invention is directed to a method for producing disposable plastic cups, lids, clam-shell type trays, and containers of the polyethylene terephthalate (PET) type wherein from 40% to 90% of the PET is regrind that has been directly recycled into the manufacturing process. This method is useful because it provides a clear and resilient cup, lid, clam-shell type tray, or container which does not crack when subjected to stress.
b. Prior Art
Numerous methods of producing disposable beverage bottles and other containers from PET, or blends of PET with other materials, such as polycarbonates, are disclosed in the prior art. However, based upon the cost of the raw starting materials, disposable articles such as plastic drinking cups, lids, clam-shell type trays,and containers are primarily made of polystyrene or polystyrene blends. Polystyrene suffers from the disadvantage in that it is brittle and cracks when subjected to stress. This is a serious disadvantage in a liquid filled cup or container that may crack and leak if squeezed too hard or impacted. As one solution to the problem, U.S. Pat. Nos. 4,386,187 and 4,386,188 (Grancio et al.) teach the use of olefin-styrene copolymer blends to produce containers having greater impact resistance than containers made from styrene alone. These copolymer blends, while tougher than polystyrene, suffer from the aesthetic disadvantage of not being very clear.
Because of cost, an even tougher plastic material, PET, which also has superior clarity over polystyrene or polystyrene blends, has been confined to use in situations wherein there is no alternative, e.g., carbonated beverage bottles. For example, carbonated beverage bottles, in addition to the requirement that they be aesthetically appealing, are required when full to withstand a 3-5 foot drop without cracking or leaking.
The thermoforming process accentuates the cost disadvantage of using PET for disposable article construction. Regardless of the plastic used, conventional thermoforming processes for any disposable cups, lids, clam-shell type trays, and containers convert approximately 50% of the starting resin sheet into uncontrollable scrap. With a high cost plastic resin, such as PET, the high cost of the scrap and its inability to be effectively recycled prohibit PETs competition with less expensive plastic resins, particularly in the highly competitive disposable goods market. The superior clarity and resilience (i.e., the ability to deform and return to shape without cracking) of PET were insufficient to overcome PET's serious cost and processing disadvantages.
The conventional teaching in the industry concerning the use of scrap PET is that a PET thermoforming resin should be virgin PET which contains no more than 20% of ground up scrap PET ("regrind PET") blended in. However, if regrind PET has been crystallized and dried by heating at 300.degree.-350.degree. F. for 3-5 hours, it can be added to the dried virgin material at up to 60% of the feedstock. ICI Films' Product Information Sheet No. 909-14, Oct. 1988!. In this latter case, the dried and recrystallized regrind material becomes the functional equivalent of dried virgin material in the thermoforming process. However, the pretreatment of scrap PET, which is generated as at least 50% of the output of a typical PET thermoforming process, by converting it into recrystallized PET, which is usable, is both an energy and time consumptive process.
Accordingly, it is an object of the present invention to provide a method for utilizing regrind PET in a thermoforming process that minimizes or eliminates the need for pretreatment of the scrap PET via recrystallization and drying.
U.S. Pat. No. 4,392,804 (J. C. Pushee et al.) discloses a method by which scrap PET is recycled during manufacture of plastic articles. Under Pushee's method, the intrinsic viscosity (IV) of the scrap PET resin must be increased by "solid state polymerization" (heating the scrap resin for 3 to 24 hours in an oven set from 172.degree.-230.degree. C.; i.e., about 341.degree. F. to about 446.degree. F., where .degree.F.=9/5.degree. C. plus 32.degree.) before it can be blended with virgin material. Like the prior art recrystallization step, Pushee's method is limited in that it requires 3 to 24 hours to convert a batch of scrap into a usable resin.
It is an object of the present invention to develop a method for utilizing, without pretreatment, the high levels of scrap PET from the conventional thermoforming process to produce thermoplastic PET sheets that are suitable for disposable article construction.
It is a further object of the present invention to utilize regrind PET to produce a disposable PET cup, lid, clam-shell type tray, or container having superior clarity and durability, and which can cost effectively compete with those objects produced by conventional plastic resins (e.g. polystyrene, polystyrene/polypropylene blends, polycarbonate and the like).
It is yet another object of the present invention to thermoform PET cups, lids, clam-shell type trays and containers from a PET mixture that is 50-90% regrind PET which would permit the utilization of regrind PET from outside sources.